The liquid crystal display uses various types of optical films such as a polarizing film and phase difference film. For example, the polarizing film of the polarizing plate used in the liquid crystal display is formed by laminating the optical film made of a cellulose film on one side or both sides of the polarizer made up of an oriented polyvinyl alcohol film as a protective film. Further, the phase difference film is used to increase viewing angle and to improve the contrast. Such a phase difference film includes the film provided with retardation by orienting the film such as cellulose ester film to provide retardation, and the film produced by coating a liquid crystal layer on a transparent substrate. It is also called an optical compensation film in some cases.
These optical films are required to be free of an optical defect and to have uniform retardation. The quality requirements are getting more and more severe, particularly due to the trend toward upsizing of the monitor and television set and improving degree of definition.
The optical film manufacturing method can be broadly classified into two types; a solution-casting film forming method and melt-casting film forming method. In the solution-casting film forming method, polymer is dissolved in a solvent and the solution is cast over the support member. Then the solvent is evaporated and, if required, the polymer is oriented to form a film. This film has been used over the wide range because of uniform film thickness and other advantages. In the melt-casting film forming method, polymer is heated and melted, and is cast over the support member where the polymer is cooled and solidified. If required, the polymer is further oriented to form a film. This melt-casting film forming method is not required to dry the solvent, and offers an advantage of using comparatively compact equipment.
The technique of manufacturing an optical film according to the melt-casting film forming method is disclosed, for example, in the following Japanese Laid-Open Patent Publication H10-10321 and the Japanese Laid-Open Patent Publication No. 2002-212312. The Japanese Laid-Open Patent Publication No. H10-10321 proposes a method of cooling the melted resin sandwiched between the cooling roll with the temperature kept at uniform level across the width and the endless belt in the form of a circular arc. The Japanese Laid-Open Patent Publication No. 2002-212312 proposes a method of cooling the melted resin by sandwiching between two cooling drums.
However, the melt from heating and melting the cellulose resin has a higher degree of viscosity than the solution from dissolving the cellulose resin in a solvent. The film produced according to the melt-casting film forming method is inferior in flatness to the film produced according to the solution-casting film forming method. To put it more specifically, this technique has the disadvantage that the die line and uneven thickness tends to occur easily.